Lithium amides, for example, lithium diisopropylamide, are widely used as a reagents in the preparation of pharmaceuticals and specialty chemicals. Lithium amides are particularly useful for the preparation of lithium acetylide compounds which are used to form acetylenic substituted organic compounds such as in steroid and fragrance intermediates. Also, they can be used to form other lithium acetylides such as propynyl lithium, cyclopentadienyl lithium, and the like. In order to form the lithium acetylide, acetylene is reacted with a lithium amide, such as lithium diisopropylamide, just prior to reacting the newly formed lithium acetylide with the ketone or other reagent in the same reactor. All of these steps are performed below 0.degree. C. It is preferable to use solutions of the lithium amides in inert solvents such as heptane, cycloheptane or toluene. Usually, it is necessary to add an ether cosolvent such as tetrahydrofuran at this point to increase the limited solubility of the reagents and the subsequently formed lithium salt of the product from the reaction with the ketone. The lithium amide may be added as a preformed solution or it may be formed in the same reactor by reacting an alkyllithium, such as n-butyllithium, with an amine, such as diisopropyl amine. In either case, the lithium amide usually exhibits lower solubility than desired for maximum reactivity and yet there is a need to minimize the amount of solvents employed.
In order to increase the concentration of the lithium amide in the preformed solutions, ethers such as tetrahydrofuran and/or complexing agents such as organomagnesium compounds have been added to increase the solubility of the lithium amide in solution. The presence of the ethers makes these solutions unstable and they decompose on standing in storage at room temperature. The presence of magnesium compounds in the reaction and subsequent workup is undesirable because the possibility of lower reactivity and lower yields of desired products plus the more difficult workup due to the presence of the formed magnesium oxide which is highly insoluble and is formed during washing.
Additionally, when tetrahydrofuran is used as the solvent, it has been found necessary to limit the amount of tetrahydrofuran to no more than one mole for each mole of lithium amide in order to minimize degradation of the system. Also, it is known that reaction of n-butyl lithium in all ethers, especially tetrahydrofuran and ethylene glycol dimethyl ether, results in rapid cleavage of the ethers at room temperature.
Mixtures of lithium diisopropyl amide and magnesium bis(diisopropyl)amide are known in a solvent of tetrahydrofuran and n-heptane. The magnesium bis(diisopropyl)amide is used to improve thermal stability and low temperature solubility.
It is preferable to use only active reagents to obtain the synergistic solubility effects of one lithium amide in the presence of another so as to obtain practical concentrations of lithium amide solutions.
The use of ethers or complexing reagents containing magnesium, aluminum, or the like results in impurities and by products which require further processing in order to remove. These impurities and by-products may cause side reactions which lower the yield.
It is also commercially desirable to be able to ship solutions that are about 2 Molar in order to minimize the amount of solvent required both from a transportation point of view and to permit a more concentrated reaction to be run when adding a preferred lithium amide in solution.
The article of Keith Smith entitled "Lithiation and Organic Synthesis", Chemistry In Britain, January 1982, pages 29-32, discloses the preparation of lithium dialkyl amides for use as lithiating agents by the reaction of organolithium reagents in aliphatic hydrocarbon solvents.
U.S. Pat. No. 3,542,512 to Honeycutt relates to the preparation of lithium amide by contacting lithium metal with liquid ammonia and then heating the mixture at a temperature above 150 degrees C in an inert liquid medium. The inert liquid medium includes aromatic compounds having a boiling point above 200 degrees C.
It is an object of the present invention to provide a lithium amide reagent composition having greater amounts of an ordinarily poorly soluble lithium amide in solution.
It is a further object of the invention to provide a process for preparing ordinarily poorly soluble lithium amides in higher concentrations in solution and in various solvents.
It is a still further object of the invention to prepare lithium amides which are to be used in selective reactions by admixing at least two different lithium amides with different reaction activities and achieve an overall reactivity which is an improvement over the sum of the two independents.
It is a yet further object of the invention to solubilize lithium amides which are considered insoluble at the time of preparation.
It is a yet still further object of the invention to prepare ordinarily poorly soluble lithium amides in situ and to utilize the resulting mixture to carry on further reactions.
It is understood that the term "poorly soluble" as used herein refers to a solubility of 0.45M or less in heptane or hydrocarbon solvents.